Calculators · Fuel

Fuel injector size calculator

Fuel injectors must deliver enough fuel for peak power without running flat out. These calculators size injectors for a target power, show how much power an existing injector can support, correct injector flow for a change in fuel pressure, and turn an injector pulse width into duty cycle. Flow is shown both in lb/hr, the usual US rating, and in cc/min, the usual European rating.

Injector size for a target power

Works out the flow each injector needs to support a given engine power. It spreads the engine’s fuel demand across all injectors and keeps a reserve by limiting maximum duty cycle, normally to about 80–85%.

Results
Flow per injector—lb/hrMinimum injector rating in pounds per hour.
Flow per injector—cc/minThe same rating in cubic centimetres per minute.

Formula
lb/hr = hp × BSFC / (injectors × duty)
cc/min = lb/hr × 453.59 / 60 / density

Power an injector set can support

Reverses the sizing calculation: from the flow rating of your injectors, it estimates the maximum engine power they can fuel at a chosen duty cycle. Use it to check whether stock injectors will cope with a tune.

Results
Supported power—Approximate crank power the injectors can supply.

Formula
lb/hr = cc/min × density × 60 / 453.59
hp = lb/hr × injectors × duty / BSFC

Injector flow at a different fuel pressure

Injector flow rises with the square root of fuel pressure. This shows the effective flow when you raise or lower pressure from the rated value — a common way to gain a little extra capacity.

Results
Effective flow—cc/minApproximate flow at the new pressure.
Change in flow—%Gain or loss compared with the rated flow.

Formula
flow₂ = flow₁ × √(P₂ / P₁)

Duty cycle from pulse width

Converts the injector opening time logged by the engine computer into duty cycle at a given RPM, for a four-stroke engine injecting once per cycle. Above about 85% the injector is close to running out of time.

Results
Duty cycle—%Share of the engine cycle during which the injector is open.
Time available per cycle—msLength of one four-stroke cycle at that speed.

Formula
cycle time = 120,000 / rpm  (ms)
duty = pulse width / cycle time × 100

Worked example: injectors for 300 hp

  1. 300 hp at a BSFC of 0.50 needs 150 lb/hr of fuel.
  2. Spread across 4 injectors at 80% duty: 150 / (4 × 0.8) = 46.9 lb/hr each.
  3. In cc/min: 46.9 × 453.59 / 60 / 0.74 = 479 cc/min.
  4. Raising pressure from 3 to 4 bar lifts a 440 cc/min injector to about 508 cc/min.

Typical brake-specific fuel consumption at full load

Engine and fuelBSFC (lb/hp·h)BSFC (g/kWh)
Naturally aspirated petrolabout 0.45 – 0.50about 275 – 305
Turbocharged petrolabout 0.55 – 0.65about 335 – 395
E85, turbochargedabout 0.75 – 0.85about 455 – 515
Dieselabout 0.35 – 0.40about 210 – 245

Related

Air–fuel ratio, lambda and fuel pump calculatorBSFC, fuel flow and engine efficiency calculatorTurbocharger airflow and power calculatorEngine airflow and volumetric efficiency calculator

About these results

Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.

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